Wolframite & Scheelite
Wolframite and Scheelite are the main ores of Tungsten.
Wolframite forms in quartz veins of granitic pegmatites, being of magmatic or hydrothermal origin. Wolframite is general term for iron and manganese tungstates being the intermediate member of the Ferberite-Hübnerite solid solution series of minerals and given the unit formula (Fe,Mn)WO4. A mineral with more than 80% FeWO4 can be termed Ferberite, and with more than 80% MnWO4 can be termed Hübnerite. The wolframite mineral forms monoclinic crystal systems, ranging in colour from grey/black (Ferberite) to through brown/red black to dark red (Hübnerite). The hardness is 4-4.5 Mohs, and the specific gravity 7-7.5.
Scheelite, CaWO4, forms a solid solution series with Powelite (CaMoO4). If a tungsten containing hyrothermal liquid is in contact with calcium containing rocks, for example limestone (CaCO3) scheelite will form. The Sheelite mineral forms Tetragonal crystal systems, the colour ranges from colourless, through white to yellow and grey. The hardness is 4.5-5 Mohs, and the specific gravity 5.9-6.1. Pure Scheelite has a blue-white fluorescence in Ultraviolet light. Increasing Molybdenum content will give progressively more yellow fluorescence.
The value of Tungsten ores is determined on a US Dollar price per Metric Ton Unit (corresponding to 10kg) of WO3 contained
Typically Russian and Chinese traded ore contains a minimum of 65% WO3 by weight; 50% WO3 by weight is the typical minimum of tungsten ore of African origin. Traded Ore should contain low amounts of CaO on analysis, the presence of which indicates a quantity of Scheelite. Molybdenum content should be kept as low as possible in all traded ores; this is of particular note for scheelite which in the natural state can contain up to several percent Molybdenum. Silicon content should be below 10% measured as Silicon (IV) Oxide. Other impurities such as Iron and Phosphorous should be at a minimum.